Adjusting the phase difference between a received code modulated signal and a generated replica code sequence can be used in particular for minimizing the phase difference between the signals, in order to align the generated replica code sequence with the received code modulated signal. This is required for example in CDMA (Code Division Multiple Access) spread spectrum communications.
For a spread spectrum communication in its basic form, a data sequence is used by a transmitting unit to modulate a sinusoidal carrier and then the bandwidth of the resulting signal is spread to a much larger value. For spreading the bandwidth, the single-frequency carrier can be multiplied for example by a high-rate rectangular-shaped binary pseudo-random noise (PRN) code sequence comprising values of −1 and 1, which code sequence is known to a receiver. Thus, the signal that is transmitted includes a data component, a PRN component, and a sinusoidal carrier component. A PRN code period comprises typically 1023 chips, the term chips being used to designate the “noise” bits of the code conveyed by the transmitted signal, as opposed to the bits of the data sequence. A chip is the smallest feature of the signal which can be individually separated.
A well-known system which is based on the evaluation of such code modulated signals is GPS (Global Positioning System). In GPS, code modulated signals are transmitted by several satellites that orbit the earth and received by GPS receivers of which the current position is to be determined. Each of the satellites transmits two microwave carrier signals, a carrier signal L1 and a carrier signal L2. The carrier signal L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier signal is modulated by each satellite with a different C/A (Coarse Acquisition) Code known at the receivers. As C/A codes, so called Gold codes are employed. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 chips, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with the navigation information at a bit rate of 50 bit/s. The navigation information, which constitutes a data sequence, can be evaluated for example for determining the position of the respective receiver.
A receiver receiving a code modulated signal has to have access to a synchronized replica of the employed modulation code, in order to be able to de-spread the data sequence of the signal. To this end, a synchronization has to be performed between the received code modulated signal and an available replica code sequence. Usually, an initial synchronization called acquisition is followed by a fine synchronization called tracking. In both synchronization scenarios, a correlator is used to find the best match between the replica code sequence and the received signal and thus to find their relative shift called code phase. The search is performed with different assumptions on an additional frequency modulation of the received signal. Such an additional modulation occurs always due to a Doppler effect and may occur further, for example, due to a receiver clock inaccuracy and/or other higher order dynamic stresses. The additional modulation can be as large as +/−6 kHz. The phase of the received signal relative to the available replica sequence can have any possible value due to uncertainties in the position of the satellite and the time of transmission of the received signal.
A correlator aligns an incoming signal with a replica code sequence with different assumptions on the code-phase. The correlator then multiplies the incoming signal and the replica code sequence elementwise and integrates the resulting products to obtain a cross-correlation value for each code-phase. If the alignment is correct, the correlation will be higher than in the case of a misalignment. Thus, the cross-correlation peak is an indication of the correct code-phase.
Once the correct replica code sequence and the correct phase and frequency of this replica code sequence has been found for a received signal in an acquisition process, phase and frequency of the replica code sequence can be kept in synchronization with the received signal by means of a tracking loop.
A GPS receiver, for example, receives satellite signals from at least 4 GPS satellites and processes them through several channels. Presently, more than 7 channels are available in a GPS receiver. Tracking loops compare in every channel the received signal with a terminal-generated replica code sequences. Each generated replica code sequences corresponds to the known C/A code employed by another one of the GPS satellites. The comparison is made by calculating the cross-correlation between the received signal and the respective replica code sequence. The value of the cross-correlation function is then processed in a discriminator, the output of which is used to detect the phase difference and/or the frequency difference between the received signal and the replica code sequence. The tracking loop then changes the frequency of the generated replica code sequence until the phase and the frequency of the received signal and the replica code sequence are matching. When the phase difference between the satellite signal and the replica code sequence is zero, this corresponds to the maximum output value of the correlator.
The control range in which the tracking loop is able to maintain or achieve a synchronization is limited though. The output of the discriminator is the phase shift and in case of a code signal, it is often related to the phase shift between the chips of the input signal and the chips of the replica code sequence. If e.g. the used discriminator is an early-late DLL (Delay Lock Loop), the usable control range is from −1.5 to +1.5 chips in a conventional tracking loop. This control range is also referred to as pull-in range.
Due to several reasons, the state of the system can “jump” or drift away from the desired one, and often it moves simultaneously out of the pull-in range of the tracking loop. For example, during a navigation with a navigation satellite terminal located in an urban area, a sub-urban area or a forested area etc., it is expected that short-term interrupts of less than tens of seconds will occur in the reception of signals on a line-of-sight. Such interrupts may be sufficient to cause phase and frequency of the replica code sequence to leave the pull-in range.
Until now, this has meant that a terminal has to activate a power consuming reacquisition process or even an acquisition process to achieve the lock in the tracking loop again.